1. Field of the Invention
The present invention relates to an image bearing member, an image formation method, a process cartridge and an image forming apparatus.
2. Discussion of the Background
In recent years, an organic photoconductor (OPC) has superseded an inorganic photoconductor in a photocopier, a facsimile machine, a laser printer and a multiple functional device thereof as an image bearing member in light of good performance and various kinds of advantages. For example, such an organic photoconductor has the following advantages over an inorganic photoconductor: (1) a wide light absorption wavelength range and a large light absorption amount with regard to optical characteristics; (2) high sensitivity and stable chargeability with regard to electric characteristics; (3) wide selection range of materials; (4) easiness of manufacturing; (5) low cost; and (6) non-toxic.
On the other hand, along with the recent advance of size reduction of an image forming apparatus, the image bearing member therein has been also reduced in size. Furthermore, due to movement toward high speed performance and maintenance-free, an image bearing member having a high durability has been strongly demanded. From this point of view, an organic photoconductor is generally soft considering that the main components of the surface layer thereof are a low molecular weight charge transport material and an inactive polymer. Therefore, when such an organic photoconductor is repetitively used in the electrophotographic process, the organic photoconductor has a disadvantage of being easily abraded under the mechanical stress caused by a development system or a cleaning system. In addition, to meet the demand of producing quality images, the size of toner particles decreases, which requires improvement of cleaning property. Thus, the hardness of rubber of a cleaning blade and the contact pressure thereof to an image bearing member increase. This results in another factor of accelerating the abrasion of the surface of an image bearing member. Such abrasion of the image bearing member invites deterioration of electric characteristics such as sensitivity and chargeability and causes reduction of image density, background fouling, etc., which leads to production of abnormal images. When such abrasion occurs locally and damages the surface of an image bearing member, obtained images have streak fouling. In the current situation, the working life of an image bearing member is limited by this abrasion or scar, meaning that replacement of the image bearing member is necessary. With regard to improvement on the durability of an organic photoconductor (image bearing member), reducing the amount of the abrasion described above is inevitable and an imminent problem to be solved in this field.
There are following technologies for improving the anti-abrasion property of a photosensitive layer: (1) using a surface layer containing a curing binder resin {refer to unexamined published Japanese patent application No. (hereinafter referred to as JOP) S56-48637}; (2) using a charge transfer polymer (refer to JOP S64-1728); and (3) using a surface layer in which an inorganic filler is dispersed (refer to JOP H04-281461).
Even an image bearing member having a cross-linking photosensitive layer in which a charge transport structure is formed by chemical bonding in the related technologies does not have sufficient properties in total.
JOP 2004-302451 describes a technology by which the drawbacks described above are significantly improved by forming the protective layer of an image bearing member by curing a radical polymerizable monomer having at least 3 functional groups with no charge transport structure and a radical polyemrizable compound having one functional group with a charge transport structure.
On the other hand, along with an increasing advance of the electrophotographic technology in recent years, the image bearing member has been demanded to have excellent characteristics. For example, producing monochrome images such as characters used to be the main function but the demand for producing color images such as photographs and the image quality thereof have been increasing these days.
However, when the image bearing member described above is used for a color electrophotographic apparatus or an electrophotographic apparatus without using discharging light, the drawbacks such as residual images, residual voltage, and a rise of voltage after irradiation occur. Especially, an image bearing member that is used for an electrophotographic apparatus without using discharging light produces images with an apparent residual image from the beginning.
The residual image tends to appear in a half tone image. Since a color image tend to be fanned by overlapping halftone images, the residual image creates a significant problem. Furthermore, the degree of a residual image color by color in the case of a color image is the same as that of a monochrome image but since the multiple color images are superimposed atop, the residual image easily stands out.
As a method of controlling such a residual image, there is a method of providing a discharging device for pre-irradiation to an image forming apparatus (electrophotographic device). However, in light of cost reduction and size reduction of the main body of an image forming apparatus, such a discharging device is not provided in many cases.
Although the anti-abrasion property of an image bearing member is improved due to the technologies described above, with an increasing demand for quality images and high durability, an image forming apparatus having an excellent combination of the image stability and the anti-abrasion property has been desired.